1. Field of the Invention
This invention resides in the field of polyacrylamide gels for use in electrophoresis, and particularly in the casting of such gels in plastic cassettes.
2. Description of the Prior Art
Polyacrylamide gel electrophoresis is widely used in biotechnology laboratories for the processing of biological samples to separate biomolecules present in the samples for identification and sometimes for quantitation and analysis. Protein mixtures, peptide mixtures, and mixtures of DNA, RNA, and fragments of DNA and RNA can all be separated on polyacrylamide gels. Slab-shaped gels are particularly useful in view of their ability to accommodate multiple separations performed simultaneously in parallel lanes, and to accommodate two-dimensional separations in which complex mixtures can be subjected to two different separation conditions in sequence. Slab-shaped gels also offer ease of detection of the separated species, both visually and by automated means, with or without removal of the gels from their retaining structures. Slab gels are commonly held in cassettes, which are transparent enclosures that provide dimensional stability to the gels, facilitate the loading of samples onto the gels, and allow electrical connections to be made so that electrophoretic procedures can be performed, all without the user touching the gel. Cassettes are also useful for holding precast gels, which can save time in the laboratory and provide both quality control and uniformity, particularly when the precast gel is prepared by a supplier rather than the user.
The typical slab gel cassette consists of two flat plates joined together along two opposing edges either by adhesives, chemical welding, sonic welding, or laser welding, with spacers at the edges to leave a slab-shaped cavity between the plates for the gel. When cassettes were originally manufactured, the plates were made of glass whose transparency and inertness made it easy to monitor the progress of the electrophoresis and to detect the bands of the separated biomolecules. Glass is expensive and fragile however and imposes limits on the design and shape of the cassette. This has led to the use of plastics, which not only avoid these problems but also offer the advantage of being readily formed to any shape by injection molding. The difficulty with plastics however is that plastics are permeable to oxygen. Cassette walls that are permeable to atmospheric oxygen expose the monomer solution from which the gel is cast to oxygen. This inhibits polymerization, and the result is often a defective gel, or at the least a high risk of nonuniformity in the porosity and density of the gel across the length and width of the slab.